Process for preparing haloketo acid derivatives [I]: ##STR1## wherein R is H or C.sub.1-6 alkyl, and X is chlorine or bromine, which comprises reacting .beta.-oxo-acid ester [II]: ##STR2## wherein R.sup.1 is C.sub.1-6 alkyl, R.sup.2 is C.sub.1-5 alkyl or OR.sup.3 (R.sup.3 is C.sub.1-6 alkyl), and X is the same as above, with nitrosating agent [IV]:ONOR.sup.4 [IV]wherein R.sup.4 is H, alkyl, halogen or SO.sub.3 H to give 7-halo-2-hydroxyiminoheptanoic acid ester [III]: ##STR3## wherein R.sup.1 and X are the same as above, followed by reacting the product with aldehyde or ketone; and intermediates therefor, and process for preparing said intermediates. Said haloketo acid derivatives are useful as intermediate for synthesis of cilastatin, which is useful as medicament, especially as dehydropeptidase inhibitor.